According to conventional power electronics technologies, a Si semiconductor element is generally used in order to control a high breakdown voltage and a huge amount of current in a voltage transformer, for example. More particularly, as a Si transistor functioning as a switch for a voltage transformer, an insulated gate bipolar transistor (IGBT), which can control a high breakdown voltage and a huge amount of current and which has a low ON-state resistance, is used. A Si-IGBT with a vertical structure is virtually always used to control as high a breakdown voltage as 600 V or more and as large an amount of current as 10 A or more. In such a vertical Si-IGBT element structure, the upper surface of the element including a gate that functions as a switch to turn the ON and OFF states of current has the same structure as a gate-channel portion of a metal-insulator-semiconductor field effect transistor (MISFET) with an insulated gate. Usually, a structure similar to a DIMISFET (double implanted MISFET) is fabricated in such a vertical Si-IGBT structure.
FIG. 7(a) is a cross-sectional view illustrating the structure of a Si semiconductor element (which may be a Si-IGBT or a MISFET) 1100. The semiconductor element 1100 is made of a silicon (Si) semiconductor. If the semiconductor element is implemented as an IGBT, the element has a structure in which an n−-drift layer 120 is stacked on a p-type Si substrate 110. On the other hand, if the semiconductor element is implemented as a MISFET, the n−-drift layer 120 is stacked on an n+-Si substrate. As can be seen from the plan view shown in FIG. 7(b), a p-body region 130 has been defined in an upper portion of the n−-drift layer 120. And on a plan view as viewed from right over the principal surface of the substrate, a p-body contact region 132 and an n+-source region 140 are defined in an upper portion of an internal part of the p-body region 130.
A source electrode 145 has been formed on the p-body contact region 132 and the n+-source region 140. On the surface of the p-body region 130, there is a channel region 151, on which a gate insulating film 160 and a gate electrode 165 have been stacked in this order. On a plan view as viewed from right over the principal surface of the substrate, there is an n-type region, which does not include the p-body region 130, on the surface of the drift layer 120. A portion of the drift layer 120 that is interposed between two p-body regions 130 will be referred to herein as a “JFET region” 121. Furthermore, on the back surface of the Si substrate 110, a drain electrode 170 has been formed. In the case of an IGBT, the source electrode 145 and the drain electrode 170 function as an emitter electrode and a collector electrode, respectively.
In this case, a “switching operation” refers to the ability to switch the ON and OFF states of current with the voltage applied between the source electrode 145 and the gate electrode 165 when a DC voltage is applied so that the drain electrode 170 has a positive potential and the source electrode 145 has a negative potential. When a voltage that is equal to or higher than a threshold voltage is applied between the source electrode 145 and the gate electrode 165 so that the gate electrode 165 has a positive potential, the channel region 151 gets depleted and becomes an inversion region. In that case, electrons are ready to move from the source region 140 toward the JFET region 121 via the channel region 151. That is to say, current flows. On the other hand, when no voltage is applied between the source electrode 145 and the gate electrode 165 (i.e., when a gate potential of 0 V is applied), electrons are not ready to move through the surface of the p-body region 130. That is why in that state, no current flows between the source region 140 and the JFET region 121 and the transistor remains OFF. Such a state in which the transistor remains OFF when the gate potential is 0V is called a “normally OFF state”. This is a characteristic that a high-breakdown-voltage power device, which should operate safely with current never allowed to flow unintentionally, must have.
This structure is sometimes modified in order to make a huge amount of current flow by adding a channel layer 150 as shown in FIG. 7(c). By introducing the n−-channel layer 150 into the interface between the p-body region 130 and the gate insulating film 160, the increase in resistance due to the movement of electrons (i.e., the ON-state resistance) can be minimized. However, a tradeoff is inevitable between the effect of reducing the ON-state resistance with the introduction of the n−-channel layer 150 and the normally OFF operation described above. That is why the channel layer should be designed carefully.
As described above, the switching operation is performed when a positive DC voltage is applied to the drain electrode 170 and a negative DC voltage is applied to the source electrode 145. If DC voltages of opposite polarities are applied (i.e., negative and positive voltages are applied to the drain electrode 170 and the source electrode 145, respectively), then the MISFET performs a diode operation. This is because a body diode 180 is formed by the pn junction between the p-body region 130 and the n−-drift layer 120 of the MISFET 1100. That is to say, the source electrode 145 makes ohmic contact with the p-body region 130 via the p-body contact region 132. That is why when DC voltages of opposite polarities are applied (i.e., negative and positive voltages are applied to the drain electrode 170 and the source electrode 145, respectively), the forward current produced by the pn junction of the body diode 180 flows between the source electrode 145 and the drain electrode 170. That is to say, if positive and negative voltages are respectively applied to the drain electrode 170 and the source electrode 145, the vertical MISFET 1100 operates as a switch to be controlled with the potential of the gate electrode 165. On the other hand, if negative and positive voltages are respectively applied to the drain electrode 170 and the source electrode 145, then the vertical MISFET 1100 functions as a diode. And when a forward current of a diode flows, a voltage that is equal to or higher than the built-in voltage (of approximately 1 V) of the Si semiconductor is generated.
In a power converter called “inverter”, which outputs AC current based on a DC voltage, if the current is output with respect to an L load with an inductance component (i.e., an inductive load) such as the winding of a motor, the voltage applied may be opposite to what is applied during the switching operation and the forward current of the body diode may sometimes flow.
Also, as for the Si semiconductor, a pseudo-Schottky diode, which performs a diode operation using the vertical MISFET structure shown in FIG. 7(c), has been invented (see Patent Document No. 1). The operation of that element may be the same as the diode operation in a situation where negative and positive voltages are respectively applied to the drain electrode 170 and the source electrode 145. However, the element is designed so that current flows mostly through the channel layer 150, not the body diode 180. That element can be designed so that the current that flows through the channel layer 150 in the reverse direction to the current that flows during the transistor operation can start to flow even if the voltage is equal to or smaller than the built-in voltage (of about 1 V) of the Si semiconductor when negative and positive voltages are respectively applied to the drain electrode 170 and the source electrode 145 and that current never passes through the body diode.
In the vertical MISFET and pseudo-Schottky diode described above, the design of the channel layer is linked to the design of the MOS interface and needs to be carried out based on a complicated principle and know-how that has been obtained from a huge amount of actually measured data. In the prior art, a channel layer with a thickness of 0.1 μm or more, n-type conductivity, and a concentration of 2×1017 cm−3 or less has been used.
As related technologies, an MOS diode that uses Si is disclosed in Patent Document No. 2 and an MOS transistor that uses SiC is disclosed in Patent Document No. 3.